Many situations exist wherein the sensing of telemetric parameters, such as distance, is desirable. Frequently, the magnitude of the parameter is sensed with an electrical implementation including a sensor (e.g. transducer) having an impedance which varies as a function of magnitude changes in the sensed parameter. Changes in the magnitude of sensor impedance may be small so that electrical noise generated by the sensing implementation can diminish resolution considerably. Moreover, it is often necessary to process the sensed signal and transmit the same over a relatively long path to receiving apparatus. Hence, it is often desirable to employ circuitry which minimizes noise and optimizes both processing and transmission of the sensed signal.
Various known implementations employing sensory transducer devices, such as reflected impedance transducer (RIT) devices, employ a capacitively tuned transducer bridge to achieve high precision linear performance. Such implementations are subject to error due to the destructive interaction and environmental sensitivity of necessary capacitive components. Additionally, such implementations typically necessitate electronics that display a high input impedance to sense and recover voltage changes. High input impedances render the device susceptible to external radiated energy influences (i.e. noise pick-up errors).
U.S. Pat. Nos. to Frick (3,975,719; 4,502,003 and 4,783,659) generally relate to circuitry for processing a current signal from a transducer or sensor. In particular, the Frick '719 patent is directed to a two-wire transmitter providing a current signal proportional to a variable reactance to be measured. In the preferred embodiment, the two-wire current transmitter comprises an input circuit, a current control circuit, and an excitation circuit. The input circuit, which includes a varying capacitor C.sub.1 and a reference capacitor C.sub.2, provides a rectified DC current signal which is substantially proportional to the expression C.sub.2 /C.sub.1 and includes zeroing and linearizing features. The current control circuit provides energization for the input circuit and allows for control of a total current signal which is communicated to the current control circuit by way of the excitation circuit. More particularly, the total current signal varies as a function of the variable reactance.
The Frick '003 patent relates to a two-wire circuit having span means in a total current control feedback loop. In the preferred embodiment, the two-wire circuit includes a power source, coupled to first and second terminals, as well as feedback amplifier means and current control means. The first terminal communicates with the feedback amplifier means and the second terminal communicates with current control means including a sensor. The span means which is coupled to the feedback amplifier means, receives the amplified feedback signal and adjusts the amplified feedback signal as desired such that the total current is controlled by the current control means as a function of at least the sensor signal and the adjusted amplified feedback signal.
The Frick '659 patent relates to a transmitter in which analog correction signals are provided based upon stored digital correction values. In one embodiment, signals from a sensor and an analog array switch are inputted to an integrator where they are integrated and combined to form a signal V.sub.s, which is related to a sense signal. A feedback signal relating to V.sub.s is communicated to the analog switch array and a microcomputer via a comparator. The microcomputer controls a D/A converter by inputting digital correction values thereto. In turn, the D/A converter provides pulse width modulated outputs having duty cycles based upon corresponding digital inputs received from the microcomputer. Outputs from the D/A are shared directly between a drive/clock, the analog switch array and the integrator.
Although the above-noted patents represent advances in the field of sensing and decoding networks, they do not adequately address the growing need for implementations which can both accommodate broadband operating frequency sensory ranges and maximize accuracy. More specifically, a need exists for a network with broadband frequency performance having the capability to obtain results that are both highly precise and accurate, as well as results that are stable over a substantial range of operating temperatures.